Receiver algorithm for the length 4 CFC

Abstract

Decoding CFC codewords in the form of sums of QPSK-modulated spread symbols in a spread spectrum communications systems using comma-free codes with elements made of sums of QPSK-modulated secondary synchronization codes.

1. A method for decoding received sums of QPSK-modulated spreading codes corresponding to elements of CFC codewords, comprising:
 (a) despreading received sums of QPSK-modulated spreading codes with each of said spreading codes;  
 (b) forming linear combinations with coefficients ±1 and ±j of the results of step (a), said combinations corresponding to possible sums as elements of CFC codewords;  
 (c) finding the maximum of said combinations of step (b);  
 (d) determining a codeword and cyclic shift from the results of step (c).  
 
   
   
     2. The method of  claim 1 , wherein:
 (a) said sums of step (a) of  claim 1  are each sums of three QPSK-modulated spreading codes.  
 
   
   
     3. The method of  claim 2 , wherein:
 (a) said sums of step (a) of  claim 2  are selected from the group consisting of the sums indicated in slot columns of  FIG. 1   b  wherein C 0 , C 1 , . . . , C 11  represent said spreading codes.  
 
   
   
     4. The method of  claim 2 , wherein:
 (a) said sums of step (a) of  claim 2  are selected from the group consisting of the sums indicated in the slot columns of  FIG. 6  wherein C 0 , C 1 , . . . , C 5  represent said spreading codes.  
 
   
   
     5. The method of  claim 2 , wherein:
 (a) said sums of step (a) of  claim 2  are selected from the group consisting of the sums indicated in the slot columns of  FIG. 7   a  wherein C 0 , C 1 , . . . , C 15  represent said spreading codes.  
 
   
   
     6. The method of  claim 1 , wherein:
 (a) said sums of step (a) of  claim 1  are in time slots of a time-division-duplex transmission.  
 
   
   
     7. A method for frame synchronization in a time-division duplex code division multiple access system, comprising:
 (a) correlating, with each of a set of synchronization codes, received linear combinations of said synchronization codes in time slots, said linear combinations elements of an alphabet for codewords of a common-free code (CFC);  
 (b) determining a codeword from the results of step (a);  
 (c) determining frame synchronization from the results of step (b).  
 
   
   
     8. The method of  claim 7 , wherein:
 (a) said linear combinations of step (a) of  claim 7  are selected from the group of linear combinations of the form b 1 c 1 +b 2 c 2 +b 3 c 3  where each of said b 1 , b 2 , and b 3  is equal to one of ±1 or ±j, and where each of said c 1 , c 2 , and c 3  is selected from said set of synchronization codes.  
 
   
   
     9. The method of  claim 8 , wherein:
 (a) said codewords of step (a) of  claim 7  have length 4; and  
 (b) said linear combinations of step (a) of  claim 8  are indicated in  FIG. 1   b  where said C 0 , C 1 , . . . , C 11  represent said set of synchronization codes.  
 
   
   
     10. The method of  claim 8 , wherein:
 (a) said codewords of step (a) of  claim 7  have length 2; and  
 (b) said linear combinations of step (a) of  claim 8  are indicated in  FIG. 6  where said C 0 , C 1 , . . . , C 5  represent said set of synchronization codes.  
 
   
   
     11. The method of  claim 8 , wherein:
 (a) said codewords of step (a) of  claim 7  have length 4; and  
 (b) said linear combinations of step (a) of  claim 8  are indicated in  FIG. 7   a  where said C 0 , C 1 , . . . , C 15  represent said set of synchronization codes.